The construction of factory-made flexible HVAC ducts is well known in the industry. These types of ducts usually comprise a helical-supported duct liner (sometimes referred to as the core or inner core) covered by a layer of fiberglass insulation, which is, in turn, covered by a scrim-reinforced PET vapor barrier or a PE-film vapor barrier. Scrim is a woven material that adds strength to a laminate construction when made a part thereof. U.S. Pat. Nos. 6,158,477 and 5,785,091 show typical constructions of factory made ducts. U.S. Pat. No. 5,785,091 teaches that the duct liner and vapor barrier can be manufactured from polymer films, particularly polyester. U.S. Pat. No. 5,526,849 discloses a plastic helical member in combination with a metal helical member and U.S. Pat. No. 4,990,143 discloses a polyester helix. United States Patent Publication No. 2007/0131299 discloses a polyester scrim used in a vapor barrier.
In the prior art, factory-made flexible HVAC ducts are typically constructed of three main components; a duct liner for conveying air, a layer of insulation for preventing energy loss through the duct wall, and a vapor barrier for holding the fiberglass around the liner while protecting the fiberglass from moisture. The duct liner is commonly constructed of a steel wire sandwiched between layers of polyester (PET) film. Other plastics and coated fabrics are also used to construct the wall of the duct liner. United States Published Patent Application No. 2010/0186846 to Carlay et al. is another example of flexible duct and it is incorporated in its entirety herein.
Another example of a prior art duct is that shown in United States Published Patent Application No. 2015-0090360 to Carlay III. This duct has an inflatable jacket to create an air space around the duct core or liner to reduce the amount of bulk insulation in the duct without reducing the overall insulating value of the duct. While this duct is advantageous in terms of its insulating value, it has some drawbacks in terms of manufacture to create the inflatable jacket.
In the HVAC industry, ductwork is often times specified to have a certain thermal resistance or R value for a particular application. For example, if the ductwork is to run in an unconditioned space, the R value must be at least 6.0. Current North American flexible duct fiberglass R-values are R4.2, R6.0 and R8.0 and each may be purchased pre-certified from fiberglass manufacturers. Obviously, the cost of the ductwork increases from one that has an R6.0 value to an R8.0 value due to the need to provide additional insulation, which is generally fiberglass insulation.
In the HVAC industry, the fundamentals of heat transfer and the like are explained in the ASHRAE Handbook of Fundamentals (the Handbook), which is currently in a 2013 edition. Included in this Handbook is the recognition of reflective insulation systems, which combines a reflective insulation and an enclosed air space bounded within a particular assembly, see page 26.12 of the Handbook. The Handbook also recognize the effect of thermal resistance as it relates to a particular size air space and the direction of heat flow, e.g. up, down, oblique up or down, etc., see pages 26.13 and 26.14. What these pages generally show is that an increase in thermal resistance occurs when the air space or air gap increases and that the thermal resistance is the least when the heat flow is in the up direction.
However, there is always a need to provide improved duct designs in the HVAC industry and other areas where air or fluid handling is necessary. The present invention responds to this need by providing an improved insulated duct.